The present invention relates to calibrating electronic equipment, and in particular to a method and apparatus for allowing faster calibration of a measuring instrument.
It is well known that analog circuitry provides operating characteristics which vary from circuit to circuit and over time. In measuring instruments (e.g oscilloscopes), in particular, it is important that such variations in operating characteristics be minimized, both from instrument to instrument and over time, in order to maintain accuracy and repeatability of the measurements. Various methods have been proposed.
Such measurement instruments were provided with signal compensation circuitry whose characteristics were specified to compensate for any variation in operating characteristics of the analog circuitry. Early solutions added analog filters to the analog measurement circuitry. The characteristics of the filters were controlled to compensate for the operating characteristic variation in the analog measurement circuitry. But these filters suffered the same variations as the analog measurement circuitry. Later solutions substituted a digital filter for the analog filter. The characteristics of the digital filters did not vary from instrument to instrument, nor over time.
Some implementations included controlled digital filters whose characteristics could be varied. This allowed each instrument to be compensated separately for the particular operating characteristics of its analog circuitry. This also allowed for the measurement instrument to be recalibrated as desired by readjusting the operating characteristic of the digital filter as the operating characteristic of its analog circuitry varied over time.
U.S. Pat. No. 4,789,952, entitled xe2x80x9cMethod and Apparatus for Digital Compensation and Digital Equalizationxe2x80x9d, issued Dec. 6, 1988 to Lo et al. illustrates such a system. In Lo et al., a training function generator provides a training signal having known characteristics to the input terminal of a signal acquisition circuit including analog and digital circuitry. The output signal from the signal acquisition circuit is supplied to an adaptive digital filter, which acts as the signal compensating circuit. Both the training signal from the training function generator and the output from the adaptive digital filter are supplied to a controller. The controller compares the training signal to the output of the adaptive digital filter produced in response to the training signal. Control parameters for the adaptive digital filter are calculated in the controller so that the combination of the signal acquisition circuit and adaptive digital filter exhibits the desired operating characteristic, i.e. so that the output of the adaptive digital filter is the desired function of the training signal. After the parameters of the adaptive digital filter are set in this manner, the training function generator and controller are disconnected from the remaining circuitry. The system then receives an input signal at the input of the signal acquisition circuit and produces an output at the output of the adaptive digital filter which will exhibit substantially the desired operating characteristic.
Methods for generating the control parameters for the adaptive digital filter in the controller (e.g. as illustrated in Lo et al.) are complex and take a relatively long time to compute. Furthermore, to increase the accuracy of the signal compensation, more samples of the training signal must be measured and processed. However, the more samples of the training signal which are measured and processed, the longer the time necessary to generate the control parameters.
When an instrument is used to make amplitude measurements of AC signals, for example, it is necessary to maintain the accuracy of the response of that instrument over a desired range of frequencies. In order to provide compensation, samples of the response of the instrument at a plurality of respective frequency locations are measured and analyzed to generate the parameters of the signal compensation circuitry. Decreasing the number of frequency locations at which the response of the instrument must be measured can significantly decrease the time necessary to generate the adaptive digital filter control parameters. However, this also decreases the accuracy of the compensation. On the other hand, response values for sufficient frequency locations to generate the adaptive digital filter control parameters to the required accuracy increases the time necessary to calculate the control parameters for the adaptive digital filter. In a production environment, in which a large volume of instruments must be tested and accurately compensated, this increased time period slows the production rate. It is desirable to decrease the time necessary to generate the parameters for the signal compensating circuitry while maintaining the accuracy of the compensation.
The inventor realized that in frequency ranges where the response characteristic of the measurement instrument is changing relatively quickly the frequency locations at which the response of the instrument is taken must be relatively closely spaced to maintain the desired accuracy of compensation. However, in frequency ranges where the response characteristic of the measurement is relatively unchanging, such frequency locations may be spaced relatively far apart, and still maintain the desired accuracy. In this manner, the number of frequency locations at which the response of the instrument must be measured may be minimized. Response values for intermediate frequency locations, where necessary to calculate the compensating control parameters, may be estimated.
The inventor also realized that in a production setting the design of the measurement instrument is fixed, and the design of the analog circuit is identical from instrument to instrument. The inventor realized that in such a setting, the response characteristics from instrument to instrument are not identical, but they are very similar. Thus, a typical response characteristic may be generated by averaging, by any of a number of known statistical methods, the response characteristics of a plurality of instruments. This typical response characteristic, in combination with the measured response values at the minimum number of frequency locations for the instrument being calibrated, may be used to generate the parameters for the signal compensating circuitry.
In accordance with principles of the present invention, a method for calibrating an instrument includes the following steps. First, a plurality of typical response values at respective frequency locations is maintained. Then the response of the instrument being calibrated is measured at a subset of the respective frequency locations in the typical response. A response value for a frequency location not in the subset of frequency locations is estimated from the typical response values and the measured response values. Then the instrument is calibrated based on the actual and estimated response values.
In accordance with another aspect of the invention, a calibrator for calibrating an instrument includes a signal generator, coupled to the instrument, which generates a signal having known characteristics. A controller is also coupled to the instrument and generates a calibration signal for the instrument. The controller includes a memory for storing a plurality of typical response values at respective frequency locations. The controller also includes circuitry which measures a plurality of response values at a subset of the respective frequency locations. An estimator in the controller estimates a response value at a frequency location not in the subset of frequency locations in response to the typical response values and the measured response values. Finally, the instrument is calibrated by circuitry in the controller which is responsive to the measured and estimated response values.